Monogamy has evolved independently in many taxa, and often involves biparental care of the young and/or low defendability of multiple mates. In many teleost fishes, however, strict monogamy is practised without such limitations. In this study, we examined why males of the pipefish Corythoichthys haematopterus (family: Syngnathidae) reproduce monogamously without changing to another mate. For this we examined the time cost associated with mate change by experimentally removing females from mating pairs and compelling the males to change mates. Mate-changing males needed longer interspawning intervals, an average of 8.5 d, than their monogamous counterparts, which was primarily because of the time needed for the new female to prepare mature eggs. As a result, we assume that mate change entails considerable reproductive costs associated with a decrease in reproductive rate. Monogamy and long-term pair bonding in C. haematopterus are likely maintained because of high reproductive rates by repeatedly reproducing with the same mate over a lifetime.